Solid composition for the pickling of stainless steels and alloys with a high content of chrome



United States This is \a continuation of application Ser. No. 297,968 filed July 26, 1963, now abandoned.

The present invention relates to a new solid composition suitable for the pickling of stainless steels and alloys with a high content of chrome as well as the respective picking process.

According to the known techniques, the pickling consists in dipping the above mentioned materials into an acid bat-h generally composed of nitric acid and hydrofluoric acid (fluonitric bath) and whose purpose is to remove and in part to dissolve the scales of the various metal oxides, which have formed on the surface as a result of the hot-working and heat-treatments of said materials, and to induce on the surface of the stainless steel a passivation layer which makes it resistant to corrosion.

The fluonitric solution is, however, rather corrosive and the treatment must be carried out with utmost care: as a matter of fact, besides the oxide scales also the metal surface itself can be etched causing pitting on same, which makes the manufactured article unsuitable for further treatments.

The development of acid fumes from the bath during the dipping of the pieces to be cleaned may cause also, on account of the gas bubbles forming within the pickling liquid following the action of the pickling solution on the metal, a corrosion of the metallic superstructures as well as irritating injuries particularly to the eyes and respiratory channels of the operational personnel in charge of the processing. A further drawback is furthermore in the handling of the liquids and in their dangerousness.

Object of the present invention is that of providing a pickling composition suitable for eliminating all the previously mentioned drawbacks, in particular a composition which, through achieving a pickling efficiency equal to that oitered by the simple fiuonitric bath, ensures a reduction of the development of the acid fumes from the bath, a reduction of and control over the corrosion, the formation on the metal of a protective film, if such is desired, and which besides is of easy handling, thereby reducing to a minimum any risks to the operational personnel and simplifying the preparation of the bath.

These and further objects, which will appear from the following description, are achieved, according to the invention, by a composition formed by a solid mixture, in the form of powder or granules, consisting of a soluble inorganic fluoride, of a soluble phosphate and, if desired, of an inorganic base capable of forming complexes with iron and the other metals such as nickel and chrome. Said mixture may be dissolved indifferently either in a diluted solution of nitric acid or, in the case of acid fluorides and acid phosphates, in water only, thus forming the bath for the complete pickling of stainless steels in general.

The soluble fluoride preferably used for that purpose is ammonium bifluoride which gives to the composition according to the invention the advantages offered by the hydrofluoric acid in dissolving the metal oxides which have formed on the surface of the steel.

The use of the soluble phosphate, particularly sodium, potassium and ammonium phosphates, monobasic and dibasic, such as for instance, disodium acid phosphate,

atent O BAWAM Patented Dec. 31, 1968 in combination with the fluoride, brings with it the following advantages:

(a) formation on the surface of the stainless steel of protective film which completes the passivating action of the nitric acid.

The protective effect originates from the capacity of the phosphoric ions of forming together with the ion a film of an unsoluble salt near a film of oxide formed by the action of the nitric acid solutions.

(b) reduction of the acid fumes that form during the dipping of the pieces to be cleaned owing to the gas bubbles developed inside the liquid, following the action of the pickling solution on the metal. In fact, the acid phosphate forms on the metal a protective film which by preventing the metal from being further attacked, limits and regulates the resulting development of gas.

(c) reduction of the corrosion of the cleaned metal surface, after the scales have been removed, in as much as the passivating action of the acid phosphate, prevents the metal from being further attacked by the bath.

(d) formation on the surfaces of the metal of an invisible film which acts as a lubricant for eventual subsequent working of the pickled pieces (cold-rolling, drawing, etc.)

When the formation of a passivating layer on the metal is not desirable (for instance in the case it is desired to proceed to a subsequent passivating treatment in concentrated nitric acid) but when all the same it is desired to achieve all the other perculiar advantages of the bath prepared with the composition object of the present invention, particularly for instance the reduced development of gas with the consequent decreasing of the formation of acid fumes and the other technological characteristics such as low volatility and absence of a foaming action, an organic base capable of forming iron complexes or complexes of other metals such as nickel and chrome, is added to the pickling composition. Organic bases which can be used according to the invention are, for example: morpholine, N-methyl-morpholine, pyridine, N-methyl-pyridine, a-picoline, fi-icoline, 'y-picoline. Particularly suited for that purpose proved to be methylethyl-pyridine in its salified form together with hydrofluoric acid.

The composition of the product, object of the present invention may vary within the following limits:

Parts, percent by weight Soluble fluoride, particularly ammonium bifluoride 99.550 Soluble phosphate, particularly soluble acid phos phate 0.5-5O Organic base (salified with hydrofluoric acid) 0-5 The composition of the invention is employed for the preparation of the pickling bath in such amount that, when dissolved in water or diluted acid, the solution will contain from about 0.5 to about 8% by weight of fluorine ion. Temperatures of the pickling bath are profitably comprised in the range of 20-70" C.

The examples quoted hereunder serve to illustrate the operational procedures according to Which the tests had been carried out and to put into evidence the advantages offered by the use of the pickling agent according to the invention.

Two types of austenitic stainless steel were taken into consideration which had the following composition:

Carbon, Chrome, Nickel, Molyb- Silica, Manga- Type No. percent percent percent denurn, percent nese,

percent percent 1 0. 059 18. 08 9. 44 0. 12 0. 44 0.85 2 O. 064 16. 70 12. 22 2. 33 0. 40 0.96

From sheets of the above materials were cut out samples of the size 100 x 120 x 2 mm., which were then heated in a muflie furnace at a temperature of 1050 C. for minutes, and thereupon rapidly cooled in the air.

Following this thermic treatment a black scale of oxides firmly anchored to the metal samples was formed on the metal. The samples thus prepared were then used for the comparative pickling tests.

From the same sheet materials were obtained further samples having a size of 70 x 20 x 2 rnm., which, after having been carefully cleaned on sand paper under water, were used for the comparative corrosion tests.

Examples 1 and 2 refer to the preceding technique; the other examples (3-6) illustrate the present invention.

Example 1 A solution containing parts by weight percent of nitric acid and as much hydrofluoric acid as to have 3 parts by weight percent as fluorine ion, was prepared as advised in the literature.

In that solution, heated at 50 C., were dipped for half their length three samples of type 1 steel, for a period of 15 minutes. Then, after that lapse of time, it was observed that the immersed surfaces were completely free of scales and were showing the normal aspect of stainless steel.

Two samples with their surfaces free of scales, not heat-treated and made of type 1 steel, were dipped into the same solution which had been used for the pickling for 16 hours at the same temperature, i.e. 50 C.

The loss in weight, due to corrosion, turned out to be on the average 2.043 grs. corresponding to about 8,300 m.d.d. (mgr./dm. x day).

The same pickling and corrosion operations were carried out on samples made of type 2 steel.

After 20 minutes the surfaces appeared clean and the corrosion turned out to be on the average 0.530 gr. corre sponding to about 2,200 m.d.d.

Example 2 clean and of good appearance, such as those obtained in Example 1. The losses in weight due to the corrosion of the specimen, free of scales, not heat-treated, and after 16 hours,

were:

for steel of type 1 of an average 1.816 grs. corresponding to about 7,400 m.d.d.

for the steel of type 2 of an average of 0.450 gr. corresponding to about 1,800 m.d.d

It should be noted that in Examples 1 and 2, during the immersion of the sample for pickling, the formation of a rather considerable amount of fumes is to be observed above the pickling baths, which is due to the bursting of the gas bubbles which originate from within the liquid and near the metal surface.

Example 3 In this example the pickling solution consists of 15 parts by' weight percent of nitric acid and of 4.59 parts by weight percent of the product object of the present invention (98% of ammonium bifiuoride and 2% of disodium acid phosphate) so as to have a solution containing 3 parts by weight percent of fluorine ion as in the preceding examples.

As for the pickling times and the aspect of the samples at the end of the operation, they are the same as those found in Examples 1 and 2.

The losses in weight clue to the corrosion of the samples free of scales, not heat-treated, and aft-er 16 hours,

were:

for the steel of type 1 of an average 1.485 grs. corresponding to about 6,000 m.d.d.

for the steel of type 2 of an average 0.294 gr. corresponding to about 1,200 m.d.d.

Example 4 In this example the pickling solution consists of 15 parts by weight percent of nitric acid and of 5 parts by weight percent of the product object of the present invention of ammonium bifiuoride and 10% of disodium acid phosphate) so as to have a solution containing 3 parts in weight percent of fluorine ion same as in the preceding examples. As for the pickling times and the aspect of the samples at the end of the pickling operation, they are the same as those found in examples Nos. 1 and 2.

The losses in weight due to the corrosion, for samples free of scales, after 16 hours, were:

for the steel of type 1 of an average of 0.753 gr. corresponding to about 3,100 m.d.d.

for the steel of type 2 of an average of 0.266 gr. corresponding to about 1,100 m.d.d.

Example 5 In this example the pickling solution consists of 15 parts by weight percent of nitric acid and 6.42% parts by weight percent of the product object of the present invention (70% of ammonium bifluoride and 30% of disodium acid phosphate) so as to have a solution containing 3 parts by weight percent of fluorine ion, same as in the preceding examples.

As for the pickling times and the appearance of the samples at the end of the operation, they are the same as those found in examples Nos. 1 and 2.

The losses in weight due to the corrosion, for the samples free of scales, not heat-treated, were after 16 hours:

for the steel of type 1 of an average of 0.639 grams corresponding to about 2,600 m.d.d.

for the steel of type 2 of an average of 0.249 grams corresponding to about 1,000 m.d.d.

As can be seen, on comparing with the results of examples Nos. 1 and 2, the extent of the corrosion decreases with the increases of the contents in phosphate.

Besides the reduced corrosion with respect to the aforementioned examples, in examples Nos. 3, 4 and 5 a reduction in the fumes hovering over the bath during the dipping of the samples was observed.

Example 6 The composition of the pickling bath is identical to that of Example 4; it contains in addition 0.10% by weight of fiuoro-hydrate of methyl-ethyl-pyridine.

By using this pickling bath, while a reduction of the acid fumes is ensured, such as in example Nos. 3-45, the disappearance of the protective film due to the action of the phosphate is provided. In consequence thereof the metal surface thus activated sutfers a more considerable attack.

The losses of weight due to the corrosion acting on the scale free, not heat-treated samples were after 16 hours:

for the steel of type 1 of an average of 1.754 grs. corresponding to about 7,100 m.d.d.

for the steel of type 2 of an average of 0.311 gr. corresponding to about 1,300 m.d.d.

We claim: 1. A process for pickling bodies of stainless steels and alloys of high chromium content wherein the bodies are heat-treated at a temperature in the range of about 20 to 70 C. in a bath formed by a dilute aqueous solution of a composition consisting essentially of 50% to 99.5% by weight of ammonium bifluoride and 50% to 0.5% by weight of an alkali acid phosphate, in an amount of said composition such that the fluoride ion concentration in said solution ranges between substantially 0.5 and 8% by weight.

2. The process defined in claim 1 in which said base is methylethylpyridine.

3. In a process for pickling bodies of stainless steel and alloys having a high chromium content, wherein the bodies are heated in an aqueous acidic bath containing nitric acid, the improvement which comprises the steps of reducing the acid fumes developed from the bath by dissolving therein a composition consisting essentially of 50% to 99.5% by weight of ammonium bifiuoride and 50% to 0.5% by weight of an alkali acid phosphate, in an amount of said composition such that the fluoride ion concentration in said bath ranges between substantially 0.5 and 8% by weight; and carrying out the heat treatment at a temperature between substantially 20 C. and 70 C.

4. The improvement defined in claim 3 wherein said alkali acid phosphate is disodium acid phosphate.

5. In a process for pickling bodies of stainless steel and alloys having a high chromium content, wherein the bodies are heated in an aqueous acidic bath containing nitric acid, the improvement which comprises the steps of reducing the acid fumes developed from the bath by dissolving therein a composition consisting essentially of 50 to 99.5% by weight of ammonium bifluoride, 50 to 0.5% by weight of an alkali acid phosphate, and a complex-forming amount up to 5% by weight of an organic base selected from the group which consists of morpholine, pyridine and picoline compounds capable of forming complexes with iron, nickel or chromium, in an amount of said composition such that the fluoride ion concentration in said bath ranges between substantially 0.5 and 8% by weight; and carrying out the heat treatment at a tem perature between 20 C. and 70 C.

6. The improvement defined in claim 5 wherein the alkali acid phosphate is disodium acid phosphate and the organic base is methylethylpyridine.

7. A composition for addition to a dilute aqueous acid bath for pickling stainless steel and alloys of high chromium content which consists essentially of to 99.5% by Weight of ammonium bifluoride, 50% to 0.5% by weight of an alkali acid phosphate, and an organic base admixed with the alkali acid phosphate and the ammonium bifluoride in a substantial amount up to 5% by weight of the composition, said organic base being selected from the group of morpholine, pyridine and picoline compounds capable of forming complexes with iron, nickel and chromium.

References Cited UNITED STATES PATENTS 2,288,995 7/1942 De Long 148-615 2,316,220 4/1943 Brown 148- .15 2,446,060 7/ 1948 Pray et al. 148-6. 5 2,665,231 1/1954 Amundsen et al. 148--6.15 3,024,148 3/1962 Schaer 1486.15 X 3,239,466 3/ 1966 Hoornstra 252-794 2,665,231 1/1954 Amundsen et al. 1486.15

RALPH S. KENDALL, Primary Examiner.

US. Cl. X.R. 

1. A PROCESS FOR PICKLING BODIES OF STAINLESS STEELS AND ALLOYS OF HIGH CHROMIUM CONTENT WHEREIN THE BODIES ARE HEAT-TREATED AT A TEMPERATURE IN THE RANGE OF ABOUT 20* TO 70*C. IN A BATH FORMED BY A DILUTE AQUEOUS SOLUTION OF A COMPOSITION CONSISTING ESSENTIALLY OF 50% TO 99.5% BY WEIGHT OF AMMONIUM BIFLUORIDE AND 50% TO 0.5% BY WEIGHT OF AN ALKALI ACID PHOSPHATE, IN AN AMOUNT OF SAID COMPOSITION SUCH THAT THE FLUORIDE ION CONCENTRATION IN SAID SOLUTION RANGES BETWEEN SUBSTANTIALLY 0.5 AND 8% BY WEIGHT. 